In recent years considerable interest has focused on development of digital video distribution systems, many of which would utilize one or another form of wireline transport of broadband signals to subscribers premises. In particular, a variety of networks have been proposed to enable local telephone companies to upgrade their networks to offer video services in competition with cable television companies.
U.S. Pat. No. 5,583,864 to Lightfoot et al. discloses several examples of such networks. The examples disclosed in that Patent include a point-to-point switched network utilizing Asymmetrical Digital Subscriber Line (ADSL) transport of switched 1.5 Mbits/s video signals from the serving end office over twisted wire pair to the customer premises equipment. In addition to these video signals, the ADSL loop from the end office switch to the customer premises carries telephone signals and upstream digital control signals relating to the video service. This ADSL network, however, does not provide enough bandwidth to offer simultaneous video services to a plurality of receivers in one subscriber premises.
Another network disclosed in the Lightfoot et al. Patent utilizes a hybrid fiber-coax network utilizing RF transport of multiplexed channel signals for broadcasting digital broadband signals from the serving end office to a number of subscriber premises connected in parallel to one segment of coax cable. The coax cable also carries two-way digital signaling channels and may carry multiplexed telephone signals. The hybrid fiber-coax network does provide the bandwidth for multiple simultaneous video sessions at each customer premises. However, installation of optical fiber and coaxial cable loop plant from end offices to large numbers of subscriber premises is prohibitively expensive, particularly if implemented by a local telephone company which cannot start from an installed base of existing coaxial cable facilities.
U.S. Pat. No. 5,583,863 to Darr et al. discloses a full service network using optical fiber transport of Asynchronous Transfer Mode (ATM) cell streams. A Host Digital Terminal (HDT), typically located at an end office, selectively switches streams containing requested programming over optical fibers to optical network units (ONUs). The ONUs in turn supply the individual streams over coaxial drop cables to customer premises equipment. The multiplexed digital bit stream transmitted over each drop cable contains selected programming for as many as three active receivers. Like the hybrid fiber-coax network, the HDT and ONU based system does provide simultaneous broadband sessions for a plurality of receivers at each premises. However, customer premises may be 900 feet or more from the nearest ONU. As a result, the coaxial cable installation all the way from the ONU to each customer premises is prohibitively expensive.
The local telephone exchange service carriers have invested very heavily in installation and maintenance of loop plant based on twisted wire copper pairs. Complete replacement of those facilities with various combinations of optical fiber and coaxial cable, while technically feasible, is not practical. Communities and home owners are very reluctant to allow such carriers to tear up streets and private properties to install new plant, and as noted, the installation of fiber and/or coaxial plant is quite expensive. The local telephone service carriers therefore have a strong incentive to utilize their existing twisted wire pair facilities and/or install new twisted wire pair facilities which are less expensive than other options outlined above.
A number of companies therefore have been developing a switched digital video (SDV) type full service network (FSN) architecture which relies more heavily on twisted pair wiring in the loop plant but provides sufficient digital broadband service to facilitate simultaneous broadband sessions for a number of terminals over the same drop cable. Although the precise transport protocols differ somewhat, the network utilizes optical fiber backbone transport, HDT based switching and optical fiber transport to ONUs similar to those disclosed in the Darr et al. Patent. The ONUs, however, multiplex downstream broadband signals into an OC-1 rate (approximately 51.84 Mbits/s) signal.
The switched digital video (SDV) implementation of the full service network (FSN) terminates optical fiber communications in an optical network unit (ONU), for example located in the basement of an apartment house or at the curb in a neighborhood. Twisted wire pair cables connect the ONU to terminals, and drop cables provide connections from the terminals to individual living units. For telephone service, the drop cable is a twisted wire pair. For digital video (broadband) services, the drop cable from the terminal to the customer premises typically is coaxial cable although some implementations may use an ADSL facility.
The telephone service is an analog telephone or ISDN type service. For broadband, however, the circuit from the ONU to the set-top terminal in the living unit carries the 51.84 Mbits/s (OC-1) rate digital signal, using carrier-less amplitude phase (CAP) modulation.
The SDV implementation of the network does allow local telephone carriers to develop a digital broadband communication system utilizing twisted wire pair plant facilities, and in some cases the carriers actually can utilize some existing loop plant. However, certain practices that telephone carriers have applied to telephone loop plant create problems effecting broadband signals carried over twisted wire pair.
In the telephone industry, twisted wire pair circuits from a central office or a subscriber line carrier unit generally are bridge-tapped along their length, to provide a line appearance in a number of different terminals located at different points along the multi-pair feeder cable. An installer can connect a subscriber's drop line to binding posts in the closest terminal, but the line appearance remains in other terminals connected to the multi-pair cable. At a later date, an installer can disconnect the first subscriber drop line from the one terminal, and connect a new subscriber's drop line from another terminal, in order to reuse the twisted wire pair connection through the feeder cable back to the central office for another subscriber.
In the full service network, some twisted wire pair circuits from the ONUs carry normal telephone services and can tolerate bridge taps. However, some of the twisted wire pair circuits from the ONUs carry broadband digital services using CAP modulation, and those services can not tolerate bridge tapping. The presence of bridge tapping, particularly extended wiring downstream of a particular subscriber's connection to twisted wire pair in a terminal, causes considerable disruptive interference effects. For example, the extending wiring adds capacitance and resistance. The extended wiring picks up considerable electromagnetic interference from external sources and may pick up cross-talk from adjacent active pairs. All of these effects disrupt the broadband digital service on the twisted wire pair.
To provide quality broadband service, it is necessary to keep the signal between the ONU and the customer premises as clean as possible. Accordingly, it is necessary to eliminate the deleterious effects of bridge tap type connections.
Today, to eliminate bridge tapping from pairs carrying digital broadband service, a splicer must manually cut the twisted wire pair to eliminate downstream taps. Subsequently, if a subscriber discontinues service from one location, the network operating company must send a splicer back to the terminal to restore the connection, if the company wants to reassign the same circuit to a new downstream customer.
Such manual bridge tap removal is labor intensive and expensive. Also, customer turnover necessitates repeated cutting and splicing operations which, over time, degrade the connectivity through the twisted wire pair. This degradation reduces the effective `life` of the loop plant and may necessitate the expense and inconvenience of early replacement.
A need therefore exists for a simple, effective way to eliminate bridge tap type connections for twisted pair wiring that may carry digital broadband services. The bridge tap removal should not require manual splicing operations, and repeated connection changes should not degrade connectivity through the twisted pair cable. Also, because of the large number of terminations required to service a large subscriber base, any mechanisms used to facilitate bridge tap removal must be relatively inexpensive per unit.